The reason Hulk refuses to emerge may not have anything at all to do with Thanos. Ironically, it may be that he's too damn angry.
During their first one-on-one dialogue scene together, Hulk tells Thor plainly he has no intention of returning to Earth. "Earth hate Hulk," he tells the thunder god. Thor and the Avengers? They're "Banner's friends." Not his.
On Sakaar, the Hulk was treated better than he'd ever been. No one even knew about Banner. People didn't run screaming from him; they cheered him. They had parades for him. They built a huge Hulk face on the planet's tallest building and gave him his own Hulk-friendly hot tub. Why would he want to go back to Earth?
Back to Earth, where he was hounded, where he had no friends, and in fact where he was likely still wanted for the death and destruction he caused during the Johannesburg rampage of Avengers: Age of Ultron. Once he was in Greenwich Village, with Tony Stark and other puny humans surrounding him, maybe the Hulk just didn't want to fight to protect the last place he wanted to be.
